From the viewpoint of safety and environmental protection, a mechanical property such as a strength and a flexibility, and a barrier property are required for a tube for transporting a fuel, a chemical liquid or a gas. In order to prevent an accumulation of static electricity caused by a friction with an inner wall of a tube and a discharge (ignition) during transport and to prevent a fuel, a chemical liquid or a gas from inflaming, a tube is used in which an electroconductivity has been imparted to the inner layer in contact with them.
As a method for imparting an electroconductivity to a resin, it is well known to knead and disperse an electroconductive filler to give an electroconductive resin. As an electroconductive filler to be kneaded into a resin, an ionic electroconductive organic surfactant, a metal fiber and powder, an electroconductive metal oxide powder, a carbon black, a carbon fiber, a graphite powder and the like are generally utilized, and a molded article having a volume resistance value of 10·1 to 1012 Ω·cm can be obtained by molding and processing an electroconductive resin composition wherein these fillers have been melted, kneaded and dispersed into a resin.
In particular, a method of adding a carbonaceous material into a resin is the most common (Patent document 1: JP-A-H07-286103 and Patent document 2: JP-A-H01-11161), and it is also known to blend an electroconductive carbon black in a thermoplastic resin including polyamide.
Although however, blending a carbon black including the Ketjen Black (the registered trademark by the Ketjen Black International Co.) and an acetylene black in 15% by mass or less allows for a high electroconductivity, these are difficult to control a dispersion into a resin and a especial formulation and mixing technologies are required to obtain a stable electroconductivity. Even if a sufficient electroconductivity is obtained, not only a processability extremely degrades but also the physical properties of an electroconductive resin composition such as a tensile strength, a flexural strength and an impact resistance strength also extremely degrade in comparison with the physical properties of an original resin free from an electroconductive filler.
Although there is also an electroconductive filler with a high aspect ratio such as a graphite powder in a flake form and a carbon fiber in a whisker form apart from a carbon black, amount of exceeding 15% by mass is required to exhibit an electroconductivity, which degrades the original properties of a resin and inhibits a moldability and electroconductivity with the emergence of deviation and orientation of fibers upon obtaining a molded article with a complex shape. There is also a problem in that carbon particles and carbon fibers readily slough away from the surface of a molded article (sloughy).
When the carbon fibers with different fiber diameters are blended in the same amount of mass, the fiber with the smaller fiber diameter is more excellent in imparting an electroconductivity because an electroconductive circuit network among fibers is easy to form. A hollow extra-fine carbon fiber, the so-called carbon nanotube has been recently disclosed, which has a fiber diameter smaller in two to three digits than that of conventional carbon fibers, and it has been also proposed to blend it into various resins, rubbers and the like as an electroconductive filler (Patent document 3: JP-A-H01-131251, Patent document 4: JP-A-H03-74465, Patent document 5: JP-A-H02-235945), which is regarded as an effective electroconductive filler solving the defects of the conventional electroconductive fillers.
These so-called ultrafine carbon fibers collectively called as carbon nanofiber or carbon nanotube can be generally categorized into the following three nanostructured carbon materials based on their shapes, configurations and structures:    (1) Multilayer Carbon Nanotube (Multilayer Concentric Cylindrical Graphite Layer)(Non-Fishbone Type);
Japanese publication of examined application Nos. H03-64606 and H03-77288
Japanese Laid-Open publication No. 2004-299986    (2) Cup Stack Type Carbon Nanotube (Fishbone Type);
U.S. Pat. No. 4,855,091
M. Endo, Y. A. Kim etc.: Appl. Phys. Lett., vol 80 (2002) 1267 et seq.
Japanese Laid-Open publication No. 2003-073928
Japanese Laid-Open publication No. 2004-360099    (3) Platelet Type Carbon Nanofiber (Card Type)
H. Murayama, T. maeda,: Nature, vol 345 [No.28] (1990) 791 to 793
Japanese Laid-Open publication No. 2004-300631.
In a (1) multilayer carbon nanotube, conductivity in a longitudinal direction of the carbon nanotube is high because electron flow in a graphite network plane direction contributes to conductivity in a longitudinal direction. On the other hand, for inter-carbon-nanotube conductivity, electron flow is perpendicular to a graphite network plane direction and is generated by direct contact between fibers, but it is believed that within a resin, since inter-fiber contact is not so contributive, electron flow by electrons emitted from the surface layer of a conductive filler plays more important role than electron flow in fibers. Ease of electron emission involves conductivity performance of a filler. It is supposed that in a carbon nanotube, a graphite network plane is cylindrically closed and jumping effect (tunnel effect hypothesis) by π-electron emission little occurs.
In an ultrafine carbon fiber having a (2) fishbone or (3) card type structure, an open end of a graphite network plane is exposed in a side peripheral surface, so that conductivity between adjacent fibers is improved in comparison with a carbon nanotube. However, since the fiber has a piling structure in which C-axis of a graphite network plane is inclined or orthogonal to a fiber axis, conductivity in a longitudinal fiber-axis direction in a single fiber is reduced, resulting in reduced conductivity as the whole composition.
The so-called carbon nanotubes described above are not satisfactory because they are difficult to be uniformly dispersed in a resin and the undispersed portion of carbon nanotubes is remained as an aggregate in a resin, which causes problems such as unspinnability (broken thread), filter occlusion at the discharge part of a molding machine, deterioration in the mechanical strengths such as the impact resistance of a molded article and deterioration of its surface appearance. For this reason, blending and mixing the especial compositions and the particular surface modification treatments are needed, for example, optimization of resin molecular weight (Patent document 6: JP-A-2001-310994), blending with modified resin, elastomer and compatibilizing agent (Patent document 7: JP-A-2007-231219, Patent document 8: JP-A-2004-230926, Patent document 9: JP-A-2007-169561, Patent document 10: JP-A-2004-231745) and surface modification treatment of carbon nanotube (Patent document 11: JP-A-2004-323738), and there is a problem that the kind, composition and the like of resins are to be restricted, thus, further improvements are demanded.